In many industrial applications, it is necessary to detect a concentration of an analyte in a sample, the sample being a liquid, gas or solid and the concentrations ranging from parts per billion to 100%. This invention relates to the remote detection and subsequent quantitative analysis that is necessary to determine the concentration of an analyte in a sample. Remote detection and analysis is typically necessary when, due to the chemical makeup of the sample or the commercial application for the analyte, intrusive or invasive detection and analysis would be either impractical or dangerous.
One method of detecting concentrations of an analyte in a sample is referred to as photoacoustic detection. In photoacoustic detection, the concentration of an analyte in a sample is detected by measuring the acoustic response of the sample after excitation by a modulated incident light of known wavelength. The magnitude of the acoustically detected signal corresponds to the concentration of the analyte within the sample. As the concentration of an analyte within the sample increases, an acoustic response of greater magnitude will be detected. By establishing a reference magnitude for a known concentration of a particular analyte in a particular sample, other unknown concentrations of the same analyte and sample can be subsequently determined.
There are a number of practical limitations associated with photoacoustic detection of an analyte in a sample, particularly if the sample is a flowing liquid or gas. For instance, it can be extremely difficult to direct a modulated incident light into a flowing sample in a manner which produces a "clean," photoacoustically detectable signal. Many attempts to provide a sufficiently clean signal involve routing of the sample through an isolated chamber and then returning the sample to its normal flow path, with optical excitation and photoacoustic detection occurring in the chamber. This enables detection of a component concentration in a liquid sample while it is in a flowing state. U.K. patent application No. 2,089,041A discloses such a device.
For many volatile and/or dangerous solutions, tapping into the sample or rerouting the flow path of the sample in this manner can be extremely cumbersome and/or expensive. However, unless the sample can be isolated in this manner, or in a similar manner, it is difficult to coordinate the direction of modulated light into the sample in a manner that will produce a clean photoacoustic response signal.
It is therefore an object of this invention to provide an improved apparatus and method for remotely directing modulated light into a sample to photoacoustically detect low concentrations of an analyte within the sample.
It is another object of the invention to provide an apparatus and method for non-invasive, accurate photoacoustic detection of the concentration of an analyte in a volatile and/or dangerous sample without requiring sample re-routing or removal.
It is still another object of the invention to provide a relatively inexpensive, improved apparatus and method for effectively and conveniently coordinating optical excitation and acoustic detection of a sample in order to quantitatively measure low concentrations of an analyte in the sample.